The air which we breath typically contains a number of pollutants, such as particulate matter, volatile liquids, gases, and vapors. Because these pollutants, to varying degrees, can be harmful when inhaled, air cleaners in the industry have been designed to remove at least some of them.
One commonly used air cleaner in the industry is designed primarily to filter out the particulate matter from the air. This filter removes the particulate matter from the air by forcing the air through a filter medium or septum, which blocks the particulate matter. The filter medium may be comprised of a variety of materials, such as glass fibers, wire screens, steel wool, animal hair or hemp fibers. This type of air filter can remove such particulate matter as dust or pollen as long as the particulate matter is larger than the openings in the filter medium or septum and thus is effectively blocked by the medium while the air passes therethrough.
Most air filters in the industry are limited in the types of particulate matter that can be removed by the physical dimensions of the filter medium. The size of the openings in the filter medium must be large enough to allow air to circulate and pass through yet small enough to obstruct the particulate matter. Thus, if the openings are made smaller than a minimum opening size, the air flow is too restricted and the air filter becomes inoperable.
Also, because the typical air filter in the industry operates by obstructing the particulate matter, the proper operation of the air filter is limited in the amount of particulate matter that can be removed from a volume of air. Once the air filter obstructs that limited amount of particulate matter it becomes blocked and any further blockage by collected particulate matter will restrict or block the flow of air through the air filter and thereby render the air filter inoperable.
Another type of air filter in the industry is the active carbon filter. The active carbon filter contains activated carbon, which has a relatively high adsorption power and is therefore rather effective at removing certain gases from the air. The active carbon filter is also effective in removing certain odors from the air in addition to removing airborne particulate matter. The active carbon filter is commonly used to remove gaseous pollutants, such as those generated by cooking, body, and tobacco odors.
The active carbon filter, however, has several disadvantages. For instance, the activated carbon particles have a tendency to break down and form fine particles, which can then become dislodged from the filter element. The activated carbon particles can also shift in location and form open passageways through the filter where the air does not contact the activated carbon particles. Further, active carbon filters generally remove only certain odors from the air and leave many other odors and harmful gases unfiltered. Even for those gases which the active carbon filter can adsorb, the gases may become re-released from the filter into the air after the passage of time if the carbon filter is not cleaned.
An air filter disclosed in U.S. Pat. No. 4,604,110 of Frazier or in U.S. Pat. No. 4,534,775 of Frazier comprises a mixture of silica gel, activated carbon, and zeolite. The disclosed air filter may additionally comprise a liquid for inhibiting the growth of microorganisms. Thus, while the Frazier air filter still operates on the principle of obstruction it is able to trap a broader range of pollutants than previous air filters.
With the mixture of elements in the Frazier filter, the activated carbon is effective against non-polar compounds such as chlorinated hydrocarbons, aromatic compounds, and general organic compounds and is very effective against carbon tetrachloride, benzene, and toluene. The zeolite, or molecular sieve, has a relative small pore size and is effective against such compounds as hydrogen sulfide, alcohols, ester, and aldehydes. The silica gel has larger pores than the activated carbon and zeolite and is effective in removing long carbon chain compounds, such as organic acids.
FIG. 1 illustrates an embodiment of the filter in U.S. Pat. No. 4,604,110 to Frazier. In this figure, a filter element 10 having a bottom portion 22 and a top portion 24 comprises a dry mixture of activated carbon particles 12, silica gel 14, and zeolite particles 16. The filter element 10 has a top layer 18 of high efficiency particulate removal paper and a bottom layer 20 of polyurethane foam. Air passes through ports 26 of the top and bottom portions. Frazier states that a liquid may be added to the mixture in order to inhibit the growth of microorganisms.
The filters disclosed in the patents to Frazier operate primarily by obstructing the particulate matter and therefore have the same disadvantages as air filters in general. Namely, such air filters are limited in the amounts and size of particulate matter that can be filtered out of the air since the filters must maintain a flow of air through the filter element. The air filters are also limited in the types of pollutants that can be removed from the air and are prone to the re-release of the pollutants back into the air.